The present invention relates to a vehicle steering control system a equipped with an auxiliary steering device for optimally assisting the steering of rear wheels of a vehicle according to each given condition, and in particular to such a vehicle steering control system which is applicable to four wheel steering vehicles.
In a front wheel steering (2WS: two wheel steering) vehicle in which only the front wheels are steered, a vehicle body side slip angle is necessarily produced for the rear wheel to have a side slip angle during a cornering maneuver. This in turn requires the vehicle body to turn around its gravitational center under a yawing moment. This however is detrimental in achieving a favorable turning response of the vehicle, and also prevents a favorable settling property of the vehicle following a yaw motion. Four wheel steering (4WS) vehicles were proposed to overcome such problems associated with more conventional front wheel steering vehicles. In a four wheel steering vehicle, the rear wheels, as well as the front wheels, are steered so as to positively control the side slip angle of the rear wheels. The four wheel steering systems have since been incorporated in a large number of commercially available vehicles.
As a control method for a four wheel steering vehicle, it was proposed in Japanese patent laid open publication No. 02-37080 to control the rear wheel steering angle by using a differential element based on the rear wheel side slip angle. However, because the side slip angle of the rear wheels while making a turn is necessarily directed inward (positive), the previous proposal may improve the stability of the vehicle as indicated in FIGS. 4 and 5 of this prior patent publication, but very little improvement was made in the vehicle response in terms of the heading response of the vehicle at an early stage of steering operation. This also translates into the fact that the property of such a four wheel steering vehicle in a high speed range is not significantly different from that of a front wheel steering vehicle.
In view of such problems of the prior art, a primary object of the present invention is to provide a vehicle steering control system which can achieve a favorable heading response at an early stage of steering operation while ensuring a high level of stability throughout an entire turning maneuver.
A second object of the present invention is to provide a vehicle steering control system which can ensure a favorable turning behavior while relying on a highly simple control logic so that a stable operation may be ensured at all times.
According to the present invention, such objects can be accomplished by providing a vehicle steering control system, comprising: a rear wheel steering device; a vehicle body side slip angular speed detecting unit for obtaining a vehicle body side slip angular speed at a point ahead of a rear axle of a vehicle body; and a steering control unit for actuating the rear wheel steering device so as to reduce a magnitude of the vehicle body side slip angular speed obtained by the vehicle body side slip angular speed detecting unit.
According to this arrangement, when there is a rise in the vehicle body side slip angle which is directed outward with respect to the turning circle of the vehicle at an early stage of turning operation, or when the outwardly directed vehicle body side slip angle sharply increases, the auxiliary steering device produces an auxiliary rear wheel steering angle which is opposite in sense to the front wheel steering angle. Thereby, the vehicle body side slip angle is at least partly canceled, and this in turn increases the yaw moment and produces a sharp heading response. As the vehicle continues the turning operation, the increased yaw moment eventually produces a tendency to spin the vehicle. This translates into a vehicle body side slip angular velocity which is directed inward with respect to the tangential line of the turning circle. Therefore, the auxiliary steering device steers the rear wheels in the same phase as the front wheels, thereby offsetting the tendency of the vehicle to go into a spin. When the vehicle is turning at a steady rate without incurring any change in the vehicle body side slip angle, the auxiliary steering device does not steer the rear wheels, and the vehicle essentially behaves as if it were a front wheel steering vehicle.
In particular, by increasing the gain for determining the amount of auxiliary steering angle with respect to the vehicle body side slip angular speed with the increase in the vehicle speed, it is possible to control the heading response and improve the stability in a high speed range at the same time. In a low to medium speed range in which the influences of inertia are relatively small, there is no opposite-phase auxiliary steering, and a high level of stability is ensured.